PLEUR0STEENID2E. 



211 



46324. Slab showing the ventral aspect of the hinder extremity of 

 the left half of the plastron ; from Swanage. The whole 

 of the meso- and hypoplastrals are shown, but the ex- 

 tremity of the xiphiplastral is broken away. 



Gunnington Collection. 



48262. 



Slab showing the imperfect shell of a young individual; 

 from the Middle Purbeck of Durdlestone Bay, Swan age. 

 Figured by the writer in the ' Quart. Journ. Geol. Soc/ 

 vol. xlv. p. 516, fig. 3. The costals and marginals (ex- 

 ceptiug the first) of the right side have been chiselled away 



Pkurostermim lullocJd. — The plastron ; from the Purbeck of Swanage. About J. 

 ig, intergular shield ; g, gular shield; hum, humeral shield; pec, pectoral 

 shield ; ab, abdominal shield ; fern, femoral shield ; an, anal shield ; e.p, 

 epiplastral bone ; entp, entoplastral bone ; hy.p, hyoplastral bone ; ms.p, 

 mesoplastral bone ; hpp, hypoplastral bone ; xp, siphiplaslral bone. 



in order to exhibit the dorsal aspect of the bones of the 

 plastron. The large size of the nuchal and the encroach- 

 ment of the first pair of marginal bones on to its anterior 

 border, characteristic of young specimens, are well shown. 

 The costal shields are relatively much wider than in the 

 adult, the marginal shields appearing to encroach more 

 on the carapace as age advances. The first vertebral 

 shield is also relatively wider than in the adult, its width 

 largely exceeding that of the other shields. Again, 



p2 



